VASCULAR ORGANIZATION IN SHOOTS OF CACTACEAE. I. DEVELOPMENT AND MORPHOLOGY OF PRIMARY VASCULATURE IN PERESKIOIDEAE AND OPUNTIOIDEAE

Primary shoot vasculature has been studied for 31 species of Pereskioideae and Opuntioideae from serial transections and stained, decorticated shoot tips. The eustele of all species is interpreted as consisting of sympodia, one for each orthostichy. A sympodium is composed of a vertically continuous axial bundle from which arise leaf- and areole-trace bundles and, in many species, accessory bundles and bridges between axial bundles. Provascular strands for leaf traces and axial bundles are initiated acropetally and continuously within the residual meristem, but differentiation of procambium for areole traces and bridges is delayed until primordia form on axillary buds. The differentiation patterns of primary phloem and xylem are those typically found in other dicotyledons. In all species vascular supply for a leaf is principally derived from only one procambial bundle that arises from axial bundles, whereas traces from two axial bundles supply the axillary bud. Two structural patterns of primary vasculature are found in the species examined. In four species of Pereskia that possess the least specialized wood in the stem, primary vascular systems are open, and leaf traces are mostly multipartite, arising from one axial bundle. In other Pereskioideae and Opuntioideae the vascular systems are closed through a bridge at each node that arises near the base of each leaf, and leaf traces are generally bipartite or single. Vascular systems in Pereskiopsis are relatively simple as compared to the complex vasculature of Opuntia, in which a vascular network is formed at each node by fusion of two sympodia and a leaf trace with areole traces and numerous accessory bundles. Variations in nodal structure correlate well with differences in external shoot morphology. Previous reports that cacti have typical 2-trace, unilacunar nodal structure are probably incorrect. Pereskioideae and Opuntioideae have no additional medullary or cortical systems.     

SHOOT TIP CULTURE OF HELIANTHUS ANNUUS—FLOWERING AND DEVELOPMENT OF ADVENTITIOUS AND MULTIPLE SHOOTS

Shoot multiplication of Helianthus annuus was optimal from half shoot apices cultured on MS media with 0.1–1.0 mg/1 benzyl adenine or kinetin. Auxins inhibited multiplication and promoted callusing. Rooting was poor and was not promoted by auxins. Flowering of multiple shoots was observed after as little as 3 wk of culture. A number of plant growth regulators and environmental conditions had no effect on flowering which supports the determinate apex theory for sunflower (Habermann and Sekulow, 1972). Adventitious shoots were induced on leaves of the multiple shoots in some inbreds.     

LEAF DEVELOPMENT AND PRIMARY VASCULAR ORGANIZATION IN SHOOTS OF ANISOPHYLLEA DISTICH A

Anisophyllea disticha is characterized by strong shoot dimorphism. Orthotropic shoots with helically arranged scale leaves produce tiers of plagiotropic shoots, while plagiotropic shoots are anisophyllous and bear dorsal scale and ventral foliage leaves arranged in a unique tetrastichous system. In this study we compare the patterns of leaf development and primary vascular organization in the two types of shoots. Orthotropic shoots have an open vascular system with five sympodia. Expansion of orthotropic shoot scale leaves occurs from P1 to P10–12, and leaf tissues mature precociously. Plagiotropic shoots have a closed vascular system with six sympodia. Leaves in ventral and dorsal orthostichies do not differ significantly in size until ca. P15, but ventral leaves are distinct histologically from the second node in an orthostichy, P4–6. Ventral foliage leaves have a diffuse plate meristem, and leaf expansion continues until ca. P30. Differentiation of ventral and dorsal leaf trace procambium parallels the divergent patterns of leaf expansion. These observations demonstrate the strong correlation among shoot symmetry, leaf development, and vascular differentiation within dimorphic shoots of one species.     

NODAL SEPTA AND THE RIGIDITY OF AERIAL SHOOTS OF EQUISETUM HYEMALE

The biomechanical role of septate nodes (points of leaf attachment with transverse diaphragms of tissue) in the otherwise hollow aerial stems of Equisetum hyemale L. is examined by means of multiple resonance frequency spectra. Resonance frequencies were determined for intact shoots, as well as the same shoots through which metal needles were inserted to destroy septa at nodes. The elastic modulus (E) of shoots, before and after surgical manipulation, was calculated. Shoots with perforated septa had E values from 17 to 32% less than intact shoots. During winter months with subfreezing temperatures, shoots with intact nodal septa accumulated water (in the form of ice) in their internodal pith canals. Values of E calculated for a shoot with intact septa and internodal water, and for the same shoot without septa do not differ significantly. Calculations indicate that the mass of internodal water is a significant fraction of the mass sufficient to deflect and buckle shoots. The added mass of internodal water is sufficient to mask the actual elastic modulus of shoots. The data indicate that nodes with septa significantly influence the flexural rigidity of shoots, but that this influence changes as a function of the quantity of water found in pith cavities.     

ONTOGENY AND PHYLLOTAXIS OF THE TERMINAL VEGETATIVE SHOOTS OF MICHELIA FUSCATA

Tucker Shirley C. (Northwestern U., Evanston, Ill.) Ontogeny and phyllotaxis of the terminal vegetative shoots of Michelia fuscata. Amer. Jour. Bot. 49(7): 722–737. Illus. 1962.—Two patterns of symmetry occur in Michelia fuscata In the lead shoots, leaves arise in a 2/5 spiral arrangement which may be either clockwise or counterclockwise. Other shoots are dorsiventrally organized; these shoots produce leaves in a modified ½ phyllotaxis in which the angle between the 2 files of leaves lies between 100° and 150°, according to the particular branch. Both types of shoot have a zonate apical meristem with a biseriate tunica a central initial zone, and a peripheral zone. The apical configuration of cells does not change appreciably during the plastochron. The flat to low-convex outline of the shoot apex is maintained by initiation of the leaves close to the summit of the apex; the diameter of the meristem diminishes greatly after such an initiation. Leaf inception in the subsurface tunica layer is followed by precocious activity of the marginal meristems which extend the stipular flanges completely around the base of the apical meristem. The stipular margins then fuse laterally and form a hood over the apex. A subapical initial meanwhile is active in the leaf blade, where it persists up to the time the leaf is 2 mm high. The most recent primordium is 300 μ high before another leaf is initiated. The vascular system of the stem is a cylindrical network of leaf traces, with 6–12 traces per leaf. The procambium develops acropetally from preexisting vascular strands in the stem below. Elements of the diverse sclereid system differ in shape in different tissues, according to the availability of intercellular space. Goebel's term “Pendelsymmetrie” is discussed with reference to apical activity in Michelia.